1. Field of the Invention
This invention relates to eyepieces for finder optical systems in single lens reflex cameras and, more particularly, to eyepieces for finder optical systems having means for adjusting the diopter as one of the components of the eyepiece is made movable along an optical axis of the finder.
2. Description of the Prior Art
For the observation of an image of an object to be photographed through the finder optical system, it has been the general practice that the object image formed on the focusing screen is refocused at a point of about -1 diopter as viewed by the eyepiece through the penta dach prism. However, those of the photographers who are either quite shortsighted, or quite farsighted, or weak in the ability of adjusting diopter have difficulty in observing the object image focused at a point near -1 diopter by the naked eye. For this reason, the conventional finder optical systems of the single lens reflex cameras were provided with diopter adjusting lenses of different rating selectively attachable to the near of the eyepiece. Because of their inability of continuous diopter adjustment and of the eye point distance shortened when observing, the use of such attachment lens could not permit a very clear observation of the field of view to be fully assured.
On the other hand, there is known a method of continuously adjusting the diopter of the finder optical system by moving the eyepiece along the optical axis of the finder. The resultant diopter change .DELTA.Dp may be expressed by the following formula: EQU .DELTA.Dp=1000..DELTA.S/fe.sup.2
where fe is the focal length (mm) of the eyepiece and .DELTA.S is the amount of axial movement of the eyepiece. In the case of the 35 mm single lens reflex camera, from the length of the optical path of the penta dach prism positioned in a space between the eyepiece and the focusing screen is determined the focal length of the eyepiece to be about 60 mm. Therefore, a desired change in the diopter calls for a large displacement of the eyepiece. To allow for this, it is required to create a large space behind the penta dach prism. For compact single lens reflex cameras, this is very difficult to achieve. For example, to effect a diopter change of .+-.1 diopter, the eyepiece must be moved about 7 mm.
As the method of minimizing the required amount of movement of the eyepiece, mention may be made of those proposed in Japanese Patent Application Laid Open Nos. Sho 53-149324, Sho 54-12653 and Sho 55-67732, and U.S. Pat. No. 4,217,048.
According to these proposals, the eyepiece is constructed with a singlet lens of negative power and a singlet lens of positive power in this order from the penta dach prism side, one of which is moved along the optical axis of the finder, while the other remains stationary, to thereby effect diopter adjustment. In the standard setting, however, the finder optical system has the various aberrations of large magnitude. Also as the diopter changes, the various aberrations are varied to large extent. Regarding the quality of the image to be observed through the finder, therefore, the prior art diopter adjustment device cannot be said to be sufficiently satisfactory.